Methane Lifetime in CMIP5 simulations

NCAR CESM Chemistry-Climate Working Group MeetingBreckenridge, CO

June 21, 2011

Arlene M. Fiore

Jasmin John, Vaishali Naik, Larry Horowitz (GFDL)Jean-François Lamarque (NCAR)

Both climate and emissions exert controls on evolution of methane lifetime

• 80-90 % of tropospheric methane loss by OH occurs below 500 mb

• ~75% occurs in the tropics[Spivakovsky et al., JGR, 2000; Lawrence et al., ACP 2001; Fiore et al., JGR, 2008]

tropopause

surface

CH

CHOHk

B

]][[ 4

4tCH4=

temperature (by 2% K-1) [OH]

+ NOx sources (anthrop., lightning, fires, soils)+ water vapor (e.g., with rising temperature)+ photolysis rates (JO1D; e.g., from declining strat O3)- CO, NMVOC, CH4 (emissions or burden)

SHORTENS WITH INCREASING: tCH4

NCAR CAM-Chem and GFDL CM3 CMIP5 Simulations

1. CMIP5 (ACC-MIP) anthropogenic forcings and emissions: -- greenhouse gases (GHG) -- emissions of aerosols and tropospheric O3 precursors

note: CH4 abundance is prescribed (not emissions) -- ozone-depleting substances (ODS)2. Meteorology-dependent lightning NOx scheme

3. Climatological isoprene emissions4. Tropospheric and stratospheric chemistry5. Historical volcanic eruptions and solar forcing

CM3 [ Donner et al., 2011]• aerosol indirect effect

• MOZART-2 mech. (trop)• AMTRAC mech. (strat)• Fully coupled ocean

CAM-Chem [Lamarque et al., 2011]

• No indirect effect• reduced mech. (trop)• MOZART-3 mech. (strat)• SSTs from AR-4 (CCSM3)

CMIP5 NOx emissions and CH4 abundances:Impacts on methane lifetime?

Anth. NO (Tg yr-1)

Methane abundance

(ppb)

Figures c/o V. Naik

t NOx OH CH4

NOx OH CH4

t

RCP8.5: CH4 OH CH4

t 1.RCP4.5: CH4 OH CH4

t 2.3.

CH4 OH CH4t

CAM-Chem and GFDL CM3 models both project increases in methane lifetime under RCP8.5

CAM-Chem RCP8.5(SSTs from SRES A2)

GFDL CM3 RCP8.5

2081-2100 vs. 2006-2025:+8.6% CAM-Chem +4.3% GFDL CM3CH4

t

NOx CH4tOH + CH4 + strat. O3

(yea

rs)

Methane lifetime evolution less consistent under other RCPs, e.g. RCP4.5

CAM-Chem RCP4.5 (SSTs from SRES B1)

GFDL CM3 RCP4.5

2081-2100 vs. 2006-2025:-2.6% CAM-Chem -13% GFDL CM3CH4

t

More warming in CM3 than CAM-Chem? Role of aerosol indirect effect in CM3 response?

(yea

rs)

Historical (1850-2005) evolution of methane lifetime:

CAM-Chem & CM3 differ in sign; CM3 increase is anthrop.

Both models show downwardtrend in recentdecades: warming (T, H2O)

GFDL ACC-MIP 2000 emissions + 1860(SST+ODS+GHG) - ACC-MIP 1860 = -13% decrease in CH4 lifetime

1. Trop O3 and aerosol precursor emission changes decrease lifetime (NOx)2. Increasing SSTs and ODS over historical should also decrease lifetime Implies a role for GHG (incl. CH4) in CM3 historical increase,

though NOx + temperature rise “wins” in CAM-Chem

Trop

osph

eric

CH

4 lif

etim

e(y

ears

)

1986-2000 vs. 1861-1880:-10% CAM-Chem Historical CH4

t

+5% CM3 Historical+5% CM3 Anthro. Forcings only

Tropical OH decreases 1910-1970 role of declining photolysis rates 1940-1970?

Next steps: Examine role of -- JO1D, lightning NOx on OH -- aerosols vs. strat O3 on JO1D

OH (1x105 molec cm-3; 30S-30N)

Strat O3 column (DU; 30S-30N)JO1D (x105 s-1; 30S-30N; below 500 hPa)

CM3 HistoricalCM3 Anthro. Forcings only

Concluding thoughts: Key drivers on methane lifetimes in CMIP5 simulations

• NCAR and GFDL models differ in terms of driving role for emissions of short-lived species vs. “climate”. ACC-MIP should aid in interpreting relative roles

• Multiple, sometimes offsetting influences from water vapor, photolysis rates, OH precursor emissions (natural and anthropogenic), temperature.

• Roles of stratospheric chemistry, aerosol-cloud interactions, and associated climate responses, require further investigation. Need carefully designed simulations for attribution Observational constraints from historical period?